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suitable when the phenyl ring in the R2 group was substituted
with p-Me (2f), p-CF3 (2g), p-COMe (2h), p-CHO (2i), and
o-CO2Me (2j) regardless of the electron-withdrawing or electron-
donating groups. The alkyl-substituted alkynes were also good
substrates, and the corresponding products 4k, 4l, and 4m were
obtained in 95%, 97%, and 94% yields, respectively. The reaction of
the substrate 2n bearing the terminal alkyne gave the corre-
sponding product 4n in 62% yield. The silver-catalyzed system
was applied to a-substituted ketones. However, even if the reaction
time was 24 h, substrate 2o was not completely transformed and
produced the product 4o in only 51% yield. Aprotic polar solvents
were examined expecting the effective solvation to promote the
generation of the enolate. As a result, DMSO was found to be
suitable as the solvent to afford the product 4o in the highest yield.
The reaction of substrates 2o–2q in DMSO afforded the corre-
sponding products 4o–4p in 73%, 55% and 47% yields, respec-
tively. For the purified products 4k–4n, as for the geometry of the
C–C double bond adjacent to the carbonyl group, Z/E isomers were
K. Nogi, T. Xu, J. Terao and Y. Tsuji, J. Am. Chem. Soc., 2012, 134,
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3 (a) J. K. Harper, A. M. Arif, E. J. Ford, G. A. Strobel, J. A. Porco Jr.,
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4 Selected transition-metal-catalyzed cyclization reactions in benzyl
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1
observed based on the H NMR spectrum (Table 2). Similar Z/E
isomerization of 1-(alkoxycarbonyl)methylenephthalan derivatives
was detected in a previous study12 which suggested that the Z/E
isomerization occurs in silica gel or under slightly acidic condi-
tions. The products 4a–4j, 4o–4q were obtained as a single isomer.
The geometries of the two C–C double bonds were suggested to be
Z isomers based on NOE experiments.13
It is noted that the silver-catalyzed system effectively promoted
the C–C bond forming reaction of the o-alkynylacetophenone
derivatives and carbon dioxide to afford the dihydroisobenzofuran
derivatives bearing a carboxyl group or a methoxycarbonyl group in
high-to-excellent yields. Furan derivatives were selectively obtained
via the 5-exo-dig cyclization. Further investigations are currently
underway in our laboratory.
5 Selected transition-metal-catalyzed cyclization reactions with nucleo-
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Financial support from Keio University Doctorate Student
Grant-in-Aid Program was gratefully acknowledged.
Notes and references
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9 See the ESI†.
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13 As for the geometry of the C–C double bond adjacent to the carbonyl
group, a trace amount of E isomer was sometimes detected by
1H NMR (Z : E = >99 : 1). For the isomerization of 1-(alkoxycarbonyl)-
methylenephthalan derivatives, see ESI†.
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11322 Chem. Commun., 2013, 49, 11320--11322
This journal is The Royal Society of Chemistry 2013